Amine antioxidants have been known and are widely used to improve the thermal-oxidative stability of synthetic ester lubricants used in the lubrication of moving parts operated at very high temperature, such as jet engines and hydraulic systems for military and commercial aircraft. In operation at high temperature in the presence of oxygen and catalytically active metals, the antioxidants are depleted. Oxidative oil degradation can create acidic by-products that degrade nearby metals and can form polymers which undesirably increase the viscosity of the lubricant. This oxidative degradation can also lead to undesirable oil insoluble sludge and deposits.
U.S. Pat. No. 3,655,559 discloses alkylated diphenylamines.
Disubstituted diphenylamines are preferred as antioxidants over monosubstituted diphenylamines because of their enhanced activity as antioxidants. Disubstituted diphenylamines are commercially available. Monosubstituted diphenylamines are not known to be commercially available in pure form because of their reduced activity. Vanlube.RTM. 848 from R. T. Vanderbilt having from about 20 to about 30 mole % of monosubstituted diphenylamines in monomer form along with a variety of other diphenylamines (nonsubstituted, disubstituted, etc.) became commercially available only as recently as 1987. Oligomeric antioxidants tend to have better performance at higher temperatures than their monomeric precursors.
It is the object of this disclosure to prepare one or more oligomeric antioxidants from at least monosubstituted diphenylamine which may be used in lubricating oils to provide better oxidation resistance.